Charge Coupled Device

ABSTRACT

A charge coupled device (CCD) is disclosed which has a semiconductor body ( 20 ) comprising polymer or oligomer semiconductor material in place of the conventional silicon. A back electrode ( 22 ) of the device is electrically coupled to the semi-conductor body through a Schottky junction, reducing the availability of holes in the semiconductor body. Shift electrodes forming a shift register are driven by negative electrical potentials and accumulations of holes in p type semiconductor material represent data.

The present invention relates to charge coupled devices (CCDs).

Charged coupled devices based upon conventional silicon semiconductortechnology are well known. The inventor has recognised that it would bedesirable to fabricate CCDs using polymer semiconductor material. Onereason for this is that the potential range of polymer based electronicproducts includes devices which require at least some memory. It isdesirable in polymer based devices to minimise the number ofinterconnect lines, which will be a key yield factor with polymer basedcircuits in the near future, and CCDs are attractive from this point ofview. They can also be very compact although this is less crucial inpolymer based devices with their large area capability.

Conventional (silicon based) CCDs utilise an inversion layer in thesemiconductor to store data but this approach is impractical with atleast some polymer or oligomer semiconductors.

An object of the present invention is to provide a charge coupled deviceusing polymer or oligomer semiconductor material.

In accordance with a first aspect of the present invention there is acharge coupled device (CCD) comprising a semiconductor body, a set ofstorage electrodes separated from the semiconductor body by adielectric, and a back electrode, wherein the semiconductor bodycomprises polymer or oligomer material and the back electrode forms aSchottky junction with the semiconductor body by virtue of which thesemiconductor body is depleted of majority charge carriers, so that whenin use the storage electrodes are charged such as to attract themajority charge carriers, they create storage sites in the semiconductorbody which can take either of a first state, in which there is anaccumulation of majority charge carriers at the site, and a secondstate, in which such an accumulation is not present at the site.

Hence where the device serves as a register or memory, it is majoritycharge accumulation which serves to encode data, rather than inversionas in conventional CCDs. It might be expected that in a device operatingby accumulation the lifetime of the second state would be unacceptablyshort, since the mobility of majority charge carriers in polymers andoligomers is typically sufficiently high. This problem is addressed byprovision of the Schottky junction at the back electrode and theconsequent depletion of majority carriers in the semiconductor body. Thelifetime of the resulting second state is finite (as it is in aconventional CCD). In the present device majority carriers can beinjected into the semiconductor body from the back electrode, but thiseffect is tolerably small because the Schottky junction is biasedagainst carrier flow in this direction. As a result the lifetime of thesecond state is compatible with device function. It is found thatdespite the presence of the Schottky back electrode junction,application of suitable electrical potential to the storage electrodescreates in the body potential wells suitable to receive and retainmajority charge carrier accumulations.

In typical embodiments the device further comprises shift electrodesarranged between storage electrodes and separated from the semiconductorbody by a dielectric, by means of which charge can be moved from onestorage site in the semiconductor body to another.

Preferably the back electrode is disposed on one side of thesemiconductor body and the storage electrodes are disposed on theopposite side.

In the preferred construction the semiconductor body is a thin layer atone face of which is the back electrode and at the other face of whichare the storage electrodes and their associated dielectric. Thesemiconductor depth in such a construction can be such that the regionof majority charge carrier depletion created by the said Schottkyjunction extends through the full depth of the semiconductor body.

Preferably the Schottky junction provides a potential barrier toinjection of majority charge carriers to the semiconductor body which is10 kT or greater, where K is Boltzmann's constant and T is the device'sintended operating temperature in degrees Kelvin. T may be taken to be300 Kelvin.

A conjugated polymer or oligomer material is preferred.

A typical device embodying the present invention further comprises adata input structure comprising an input electrode arranged adjacent astorage site in the semiconductor body to cause injection of majoritycharge carriers thereto.

In one such embodiment the input electrode forms a Schottky junctionwith the semiconductor body and the data input structure furthercomprises a transfer electrode adjacent the input electrode, such thatapplying to the transfer electrode a charge opposite to that of themajority charge carriers in the semiconductor body causes injection ofmajority charge carriers to a potential well formed in the semiconductorbody by the transfer electrode. Such an arrangement is well suited toserial input of data to a CCD serving as a register or stack.

In another such embodiment data is encoded by provision of inputelectrodes adjacent to selected storage electrodes, so that uponinitialisation an accumulation of holes is injected to the storage sitescorresponding to the selected storage electrodes and not to others. Thistype of construction is well suited to applications where data ispermanently encoded in the device so that it provides a read only memoryfunction. The input electrodes may be connected to a common electricalline so that the device is initialised by applying an electricalpotential to the line in order to drive majority charge carriers intothe selected storage sites.

With current polymer materials p type polymer or oligomer material isfavoured for the semiconductor body, the device being adapted to bedriven by application of negative potentials to the storage electrodescreating sites for hole accumulation in the semiconductor body.

Preferably alternating storage and shift electrodes are arranged to forma line along which majority charge carrier accumulations are passed inuse. The line of electrodes may be addressed through first and secondelectric shift lines and preferably comprises a series of electrodepairs each comprising a lower field shift electrode electricallyconnected to an adjacent higher field storage electrode, alternate suchelectrode pairs being electrically connected to the first and secondshift lines respectively, such that by changing from time to time whichof the shift lines is at greater electrical potential, accumulations ofmajority charge carriers are passed along the line of electrodes.

The storage and/or shift electrodes may be formed by a plurality oflocalised metal layers which are anodized to form the dielectric bywhich they are isolated from the semiconductor body. The semiconductorbody may comprise a layer of polymer or oligomer deposited over themetal layers.

In accordance with a second aspect of the present invention there is amethod of manufacturing a charge coupled device comprising

forming upon a substrate a first localised metal layer to serve as afirst set of electrodes,

anodising the first metal layer to form an oxide layer upon it;

forming a second localised metal layer to serve as a second set ofelectrodes,

anodising the second layer to form an oxide layer upon it;

forming over the metal layers a semiconductor body of polymer oroligomer material and

forming upon the semiconductor body a metal back electrode, the materialof the back electrode and of the semiconductor body being such thattogether they form a Schottky junction by virtue of which thesemiconductor body is depleted of majority charge carriers.

Specific embodiments of the present invention will now be described, byway of example only, with reference to the accompanying drawings inwhich:-

FIG. 1 represents in schematic form a section through a charge coupleddevice of existing type;

FIG. 2 represents in schematic form a section through a charge coupleddevice embodying the present invention;

FIGS. 3 and 4 are band diagrams relating to the device illustrated inFIG. 2;

FIGS. 5 and 6 are schematic sections through an arrangement ofelectrodes used for data input in the device of FIG. 2;

FIGS. 7 a-e are schematic sections through the same device at successivestages in its manufacture; and

FIG. 8 is a schematic illustration of a further device embodying thepresent invention, viewed along a direction perpendicular to the devicesubstrate.

In the conventional CCD of FIG. 1, p type silicon semiconductor material2 is sandwiched between a back contact 4, which forms an ohmic contactto the semiconductor, and a dielectric oxide layer 6. Beneath the oxidesurface, and so insulated from the semiconductor, are shift electrodes8, 8′ and storage electrodes 10, 10′, the latter being closer to thesemiconductor than the former. Connections to the electrodes are madethrough first and second lines φ and φ′. In the illustration, data flowsin the direction toward the right hand side.

Alternate shift electrodes 8 are connected to the first line φ. To theright of each of these shift electrodes is a storage electrode 10 whichis also connected to the first line φ. Remaining shift electrodes 8′ areconnected to the second line φ′ and to the right of each of these shiftelectrodes is a storage electrode 10′ which is also connected to thesecond line φ′. If a positive potential is applied to any of theelectrodes it serves to create, in an adjacent region of thesemiconductor, a potential well in which electrons can be stored. Hencein FIG. 1 (relating to a conventional device based on p type silicon) aslug of electrons forming a localised inversion is seen at 12 in thevicinity of a particular electrode 10 and represents a logic “1”, whilethe absence of a corresponding inversion in the vicinity of the otherupper electrodes represents logic “0”s. Thus a register is formed forstorage of digital data but this storage is “dynamic” in the sense thatthe data can only be stored for a finite time . This is because the ptype silicon is capable of generating electron-hole pairs and theelectrons are attracted to the positive potentials existing belowregions representing logic “0”s. The corresponding holes leave thesemiconductor via the back contact. Over time all “0”s are therebyconverted to “1”s—that is, an inversion layer forms in the vicinity ofeach electrode. Storage times for the best silicon CCDs are of the orderof milliseconds.

To appreciate how data is shifted along the register, consider thesituation illustrated in FIG. 1 wherein the first line φ is initially ata positive potential with the second line φ′ at lower potential, theslug 12 of electrons being thus attracted to the adjacent storageelectrode 10, which, because it has a thin dielectric separating it fromthe silicon, creates a high field in it. If a sufficiently largepositive pulse is then applied to the second line φ′ then the relatedsemiconductor surface potential draws the electrons to the right,firstly to the adjacent shift electrode 8′ and then (since it creates alarger field) to the storage electrode 10′. In this way all of the datain the register is shifted simultaneously one place to the right.Returning the second line φ′ to its original lower potential causes theelectrons and the data again to move to the right and the shift processcan thus be repeated by applying and removing the high potential onsecond line φ′. Thus by suitable “clocking” of the electrode potentialsdata can be caused to move along the array.

In practice a typical CCD comprises a square array of electrodes anddata passes in serpentine fashion down the array. One data bit isrepresented by an adjacent set of four electrodes—two shift electrodes8, 8′ and two storage electrodes 10, 10′—so that even if the datacomprises a series of logic “1”s, this is represented by a series ofinversions over storage electrodes 10′ with no inversions over theintermediate electrodes 10. This separation of the inversion regionsensures that during the shift process an “empty” storage electrode isalways available to capture the incoming bit. Other methods of changetransfer are possible and the arrangement of electrodes correspondinglydiffers from one device to another.

Implementing a CCD which uses a polymer semiconductor in place of theconventional silicon is challenging. The best performing polymersemiconductors are typically p type. It is not possible to utilise aninversion layer to store data in such materials, however, (as in thesilicon based CCD) because in typical polymers the electrons which wouldbe required to form the inversion are immobile. There is the alternativeof storing data by use of an accumulation layer but this too isproblematic since holes forming an accumulation layer in p type polymerstend to be too mobile, more so even than those in the body of thepolymer. If the type of device illustrated in FIG. 1 were adapted tofunction in this way, replacing the silicon with p type semiconductorand applying negative potentials to the electrodes to create a negativepolymer surface potential, a functional CCD would not result. Due totheir high mobility, holes would very rapidly accumulate in thepotential wells created by the electrodes and all locations in thedevice would, in an unacceptably short space of time, change to thelogic “1” state.

FIG. 2 illustrates a CCD constructed in accordance with the presentinvention. A body of polymer semiconductor material 20 is sandwichedbetween back contact 22 and dielectric (oxide) layer 24 and in thisexample is a p type material. The polymer material chosen for thisembodiment is poly-3-hexylthiophene. Other polymer (or oligomer)materials may be used. The back contact 22 of the FIG. 2 embodiment doesnot form on an ohmic contact with the semiconductor 20. Instead it formsa Schottky barrier. In the present embodiment this is achieved byselecting aluminium for the back contact. Other metal/polymercombinations could be used, as the skilled person will recognise. Forinstance other polythiophenes may be used. The Schottky junction createsa depletion (space-charge) region within the adjacent semiconductor. Theconcentration of charge carriers (holes, in the present example) isreduced to a very low value in the depletion region, which extendsthrough the full depth of the semiconductor. The Schottky junction isalso biased against injection of holes into the semiconductor. Theresult is that the rate of supply of holes into the potential wells atthe semiconductor/dielectric junction is reduced by several orders ofmagnitude.

The main part of the register utilises an electrode arrangement of thetype already described with reference to the conventional CCD of FIG. 1,with paired shift and storage electrodes 8, 10 and 8′, 10′. As before,the register electrodes along with the oxide layer and semiconductoreffectively form MOS (metal/oxide/semiconductor) capacitors. In the FIG.2 device, however, the potentials applied to the electrodes arenegative. A logic “1” is represented by an accumulation of holes at agiven location in the register. A logic “0” is represented by theabsence of such accumulation. By virtue of the Schottky barrier at themetal/semiconductor junction of the back contact and the consequentreduction in hole availability, the lifetime of a “0” state issufficiently long to allow a functional device to be created.

FIGS. 3 and 4 are band diagrams for the FIG. 2 device with regionsrepresenting the back contact 22, polymer 20, dielectric 24 and metalelectrodes 8. The upper edge of the conduction band and lower edge ofthe valence band are indicated at 30 and 32 respectively, and the Fermilevel is represented by a line EF. Comparing FIG. 3, which shows thesituation with zero field applied by the electrodes 8, against FIG. 4,which represents the case where electrodes 8 provide a negative electricfield, it can be appreciated that the effect of an applied field is tocreate, in a region 34 of the polymer adjacent the dielectric layer 24,a potential well for holes. The potential barrier to injection of holesfrom the back contact is also seen at the polymer/back contactinterface.

It is necessary to provide for input of data to the shift register. Therelevant arrangement is seen in simplified form in FIG. 2 and in moredetail in FIGS. 5 and 6, in which the polymer semiconductor body isagain labelled 20 and the dielectric layer is 24. A metal inputelectrode 40 is provided at one end of the register, being arranged inthe oxide layer but in contact with the polymer material 20, and soforming with the polymer a Schottky diode. The input electrode 40 has 0Vapplied. Adjacent to it is a transfer electrode 42, seen in FIGS. 5 and6 to have a lower limb 44 extending beneath the input electrode. Apotential V_(in), applied to the transfer electrode 42 is varied betweenzero and a negative potential −V_(T), in synchronism with the clockingof the shift register, to write data. FIG. 5 shows the situation whenV_(in)=0. Dotted region 44 is the depletion region around the Schottkydiode. In this state holes are not transferred to the vicinity of thetransfer electrode and a logic “0” is written. FIG. 6 shows thesituation when V_(in)=−V_(T). The depletion region 44 is reduced in sizein the vicinity of the input electrode to the point where holes cantunnel into the region above the transfer electrode 42, writing a logic“1”. The holes then transfer into the register automatically as it isclocked.

A Schottky diode, arranged at the end far end of the register from theinput structure and formed in similar manner to it, is used to read datafrom the register. A clock pulse is applied to the metal electrode ofthe reader diode and logic “1” is recognised as a current flow out ofthis electrode, logic “0” being recognised by the absence of suchcurrent.

FIGS. 7 a-e illustrate the steps involved in manufacture of the polymerbased CCD. This process begins with a substrate formed in this case as acleaned plastics sheet 50. Aluminium is evaporated onto the substrate,photoengraved to form stripe electrodes 52, and then the metal surfaceanodized to form a high K dielectric (alumina) layer 54. A secondaluminium layer is then deposited and a further set of stripe electrodes56 is defined in it, each of the further electrodes 56 sitting between,but overlapping, adjacent first electrodes 52 on either side (FIG. 7 c).The further electrodes 56 are then anodized to form a second oxide layer58 which is much thicker than the first. Electrical connections to thedifferent sets of electrodes can be made on the substrate on oppositesides of the array. Polymer 60 is applied by spin coating, casting orprinting (FIG. 7 c). Lines 62 and 64 represent in schematic form theconnections to the electrodes although these connections lie outside theplane of the diagram.

The embodiment described above is essentially a register or “stack”which allows data to be input serially and subsequently output serially.CCDs can serve other purposes, however. FIG. 8 illustrates a CCDconstructed in accordance with the present invention which serves as aread only memory. The device is seen in plan (i.e. along a directionperpendicular to the substrate). Shift and storage electrodes are oncemore designated 8, 8′ and 10, 10′ respectively and are connected in thesame pattern as before to electric lines φ and φ′. However in place ofthe serial data input arrangement illustrated in FIGS. 5 and 6, thememory of FIG. 8 has a set of write contacts 80 associated with selectedshift electrodes 8. The write contacts 80 are overlapped by theircorresponding shift electrodes and may form ohmic or Schottky contacts.To read data permanently stored by the device, it is first initialisedby application of a voltage pulse to all of the write contacts 80through an input line φ_(i) which drives charge carriers (in the presentembodiment, holes) into the polymer below the selected shift electrodes8, from where it passes to the vicinity of the adjacent storageelectrode 10 and there represents a logic “1,” condition. Where no writecontact 80 is provided no charge accumulation takes place and thecorresponding storage electrode 10 carries a logic “0”. By clockinglines φ and φ¹ as before, the relevant binary number (1101 in theillustrated example) can be read through a serial output 82 as before.

The polymer based CCD of the present invention has numerous potentialapplications. One example is in the field of electromagneticallyreadable tags, whose economical manufacture from polymer materials ishighly attractive commercially. An identification code could be storedin such a tag by a ROM of the type illustrated in FIG. 8.

The above embodiments serve as examples only and should not be taken tobe limiting upon the scope of the invention. For example while only ptype devices are discussed above, the present invention is potentiallyapplicable to n type devices in which the Schottky junction formed bythe back electrode serves to deplete the semiconductor body of mobileelectrons.

1. A charge coupled device (CCD) comprising a semiconductor body, a set of storage electrodes separated from the semiconductor body by a dielectric, and a back electrode, wherein the semiconductor body comprises polymer or oligomer material and the back electrode forms a Schottky junction with the semiconductor body by virtue of which the semiconductor body is depleted of majority charge carriers, so that when in use the storage electrodes are charged such as to attract the majority charge carriers, they create storage sites in the semiconductor body which can take either of a first state, in which there is an accumulation of majority charge carriers at the site, and a second state, in which such an accumulation is not present at the site.
 2. A CCD as claimed in claim 1 which further comprises shift electrodes arranged between storage electrodes and separated from the semiconductor body by a dielectric, by means of which charge can be moved from one storage site in the semiconductor body to another.
 3. A CCD as claimed in claim 1 wherein the back electrode is disposed on one side of the semiconductor body and the storage electrodes are disposed on the opposite side.
 4. A CCD as claimed in claim 1 wherein the semiconductor body is a thin layer at one face of which is the back electrode and at the other face of which are the storage electrodes and their associated dielectric.
 5. A CCD as claimed in claim 4 wherein the region of majority charge carrier depletion created by the said Schottky junction extends through the full depth of the semiconductor body.
 6. A CCD as claimed in claim 1 wherein the said Schottky junction provides a potential barrier to injection of majority charge carriers to the semiconductor body which is 10 kT or greater, where K is Boltzmann's constant and T is the device's intended operating temperature in degrees Kelvin.
 7. A CCD as claimed in claim 1 wherein the back electrode is metal.
 8. A CCD as claimed in claim 1 wherein the polymer or oligomer material is conjugated.
 9. A CCD as claimed in claim 1 wherein the semiconductor body comprises poly-3-hexylthiophene.
 10. A CCD as claimed in claim 1 which further comprises a data input structure comprising an input electrode arranged adjacent a storage site in the semiconductor body to cause injection of majority charge carriers thereto.
 11. A CCD as claimed in claim 10 wherein the input electrode forms a Schottky junction with the semiconductor body and the data input structure further comprises a transfer electrode adjacent the input electrode, such that applying to the transfer electrode a charge opposite to that of the majority charge carriers in the semiconductor body causes injection of majority charge carriers to a potential well formed in the semiconductor body by the transfer electrode.
 12. A CCD as claimed in claim 10 in which data is encoded by provision of input electrodes adjacent to selected storage electrodes, so that upon initialisation an accumulation of holes is injected to the storage sites corresponding to the selected storage electrodes and not to others.
 13. A CCD as claimed in claim 12 wherein the input electrodes are connected to a common electrical line so that the device is initialised by applying an electrical potential to the line in order to drive majority charge carriers into the selected storage sites.
 14. A CCD as claimed in claim 1 wherein the semiconductor body comprises p type material and is adapted to be driven by application of negative potentials to the storage electrodes creating sites for hole accumulation in the semiconductor body.
 15. A CCD as claimed in claim 1 wherein alternating storage and shift electrodes are arranged to form a line along which majority charge carrier accumulations are passed in use.
 16. A CCD as claimed in claim 15 wherein the line of electrodes is addressed through first and second electric shift lines and comprises a series of electrode pairs each comprising a lower field shift electrode electrically connected to an adjacent higher field storage electrode, alternate such electrode pairs being electrically connected to the first and second shift lines respectively, such that by changing from time to time which of the shift lines is at more negative electrical potential, accumulations of majority charge carriers are passed along the line of electrodes.
 17. A CCD as claimed in claim 1 wherein the shift electrodes are formed by a plurality of localised metal layers which are anodized to form an oxide layer which is the dielectric by which they are isolated from the semiconductor body.
 18. A CCD as claimed in claim 17 wherein the semiconductor body comprises a layer of polymer or oligomer deposited over the metal layers.
 19. A CCD as claimed in claim 1 connected to clocking circuitry which applies clocked negative potentials to the storage electrodes.
 20. A method of manufacturing a CCD comprising: forming upon a substrate a first localised metal layer to serve as a first set of electrodes; anodising the first metal layer to form an oxide layer upon it; forming a second localised metal layer to serve as a second set of electrodes; anodising the second layer to form an oxide layer upon it; forming over the metal layers a semiconductor body of polymer or oligomer material and forming upon the semiconductor body a metal back electrode, the material of the back electrode and of the semiconductor body being such that together they form a Schottky junction by virtue of which the semiconductor body is depleted of majority charge carriers. 